The Invasive Catfish That Walks Across Parking Lots

You pull into the shopping center parking lot, barely noticing the usual sights. A wayward shopping car. A few pigeons. Someone pushing a stroller. A school of fish. Wait. What?

You look again, and sure enough, there’s a group of fish congregated around a puddle. You note the familiar whiskers of catfish. This shopping center is miles from the nearest river, and yet these fish appear to be alive and well. They might even be snarfing up some worms out of the puddle.

Your day is about to get even weirder. Because those catfish suddenly begin walking across the parking lot. Yes, walking.

They make their way across the parking lot, and suddenly dip down into a storm sewer. After a few minutes, you start to question what you saw. Have recent dystopian worries gotten the better of you? Will anyone believe this?

Noah Bressman will. He knows that catfish walking across parking lots isn’t out of the realm of science fiction. It’s happening in Florida. Bressman, currently a postdoctoral research fellow at Chapman University, researches amphibious fish – fish that can move on land.

His latest publication, in the Journal of Fish Biology, explores how walking catfish move about and feed on land. This fish, an invasive species in Florida, is even weirder than it appears.

A Fish Out of Water

The walking catfish (Clarias batrachus) is a member of the Family Clariidae which consists of air breathing catfish native to Asia and Africa. These catfish have a well-known ability to move across land, although little is known of how they behave terrestrially.

The walking catfish is under increasing threat in its native Asian waters. However, it is thriving outside its native range, particularly in Florida. Likely a combination of aquarium pet releases and aquaculture escapes, the walking catfish is now found widely in the state in canals, wetlands and golf course ponds. It is particularly common in the Everglades region.

The walking catfish, as its name suggests, can move quite well on land. It can cover up to 1.2 kilometers and can survive up to 18 hours out of water. And this tends to freak out some people.

Noah Bressman knows well the reactions that “walking fish” can cause. His fascination began as an undergraduate at Cornell, when he noted a mummichog – a small, East Coast fish species – on the floor of his lab. The fish had leapt out of a tank, probably to escape a predator. But on the floor of the lab, it looked like it had tried to move towards a shiny surface on the floor.

The next day, Bressman saw another mummichog in the same spot. “I hypothesized the fish were making deliberate movements on land,” he says. “They were using vision to find a way back to water. The shiny surface was giving them a cue that it was water.”

This launched his doctoral research at Wake Forest University on amphibious fishes, fish that can survive out of water. One of his research subjects is perhaps the most notorious of “walking fish,” the snakehead. While there are many more devastating invasive fish species, the snakehead grabs media headlines. It has been dubbed the “frankenfish” and spawned (sorry) a sub-genre of cheesy horror movies.

“People think of fish as completely aquatic animals,” Bressman says. “They don’t pay much attention to a frog on land, even though frogs spend a lot of time in the water. Millions of years ago, fish ventured onto land and evolved. But it is not what we think of fish. When people see fish on land now, they see a monster. Anything that shifts a paradigm, that calls into question how we see the world, is scary. That’s true whether we’re talking about politics or nature.”

The walking catfish may not have the horror-movie draw of the snakehead, but it is well-documented moving across land in Florida. “Walking catfish seem to appear out of nowhere in places you wouldn’t expect,” says Bressman. “These places are often far from any water body.”

Many amphibious fish, like snakeheads, use vision to move from one water to the next. But the walking catfish has small, poorly developed eyes, and also has been seen moving at night. How does it move, and why?

Scent of an Earthworm

Why walking catfish move on land may be easier to answer than how. Like other amphibious fish, walking catfish will “walk” to disperse to new water bodies. They have been observed laying eggs in shallow puddles where young will be safer from predators.

And walking catfish eat terrestrial prey like earthworms and other invertebrates. While they need water to swallow, they can easily feed on invertebrates in puddles and moist spots of ground.

Bressman hypothesized that walking catfish found waterways and prey via chemoreception, using chemical signals like taste and smell. He captured walking catfish in the field and tested their chemoreceptive abilities in the lab. He did this while at graduate student at Wake Forest, in collaboration with the University of Florida.

His results are published in the Journal of Fish Biology, and are the first documented instance of a fish using chemoreception on land.

Lab results found that the walking catfish oriented towards natural pond water but moved away from toxic compounds like hydrogen sulfide. The catfish didn’t orient towards distilled water.

While the catfish clearly detected organic compounds that they used to orient, the exact mechanism for this is not known. Bressman says more study is necessary. One possibility is that the catfish use their whiskers, which are essentially covered in tastebuds.

Other non-amphibious catfish are well known for their ability to detect food via their whiskers and a well-honed sense of smell in water. Catfishing often involves using extremely pungent baits to lure in the fish. One possibility is that walking catfish possess a similar ability on land.

“The walking catfish whiskers don’t droop; they can wave them around in the air,” says Bressman. “They may be tasting the compounds in the air. But we need to do more research to determine what the actual mechanism is.”

Walking Catfish in the Parking Lot

When beginning his research, Bressman realized that very little was known of the natural history of walking catfish on land. “There is still so much we don’t know about fish, including their natural history,” he says.

Despite their abundance, tracking down a fish in swamps and canals was unlikely to yield enough behavioral observations. So Bressman turned to crowdsourcing. He asks for detailed observations from citizen-scientists participating in Florida wildlife social media groups.

He gathered 88 observations and also watched YouTube videos documenting walking catfish behavior. The walking catfish is one bizarre fish, behaving like a creature created for a Carl Hiaasen novel.

Most walking catfish were documented after a rain, moving over moist environments, although one detailed report included a large school of walking catfish moving from a golf course pond to an isolated cypress pond – in broad daylight.

A biologist saw more than 100 walking catfish moving across a dirt road. There were heavy rains and a large number of earthworms were on the road. The biologist wasn’t conducting research; he was gathering dinner. He picked up about 50 catfish and carried them home for a fish fry. When he gutted them, he found half had stomachs stuffed with the same worms on the road.

“It’s clear that walking catfish feed on terrestrial prey,” says Bressman.

And then there are the parking lots. A number of observations and videos show catfish out and about in parking lots, often far from water.

One citizen scientist found them in a Pay-Less Shoes parking lot, and saw them heading back towards a storm drain. “A lot of the observations occur near storm drains,” Bressman notes.

Recently, a number of angling personalities have fished South Florida storm drains, often filming the stunt. They catch native largemouth bass and non-native peacock bass. But the bass never show up in parking lots.

“Some might at first think that the walking catfish just wash out of the storm drain into the parking lot, but you never see peacock bass or other fish that live in drains,” Bressman says. “What is more likely is that rains raise the storm drains, then the walking catfish use this to access parking lots and other areas to forage for prey.”

He believes this is a significant factor for invasive species managers to consider. “In general, dispersal by land is overlooked as a way amphibious fish spread,” he says. “It’s known that walking catfish can cover 1.2 kilometers over land. However, use of storm drains increases the potential to expand into new habitats significantly.”

Sometimes, this dispersal is aided by well-meaning people. “People see a fish out of water and think it’s in trouble,” he says. “So they drop it off in a different water body. A lot of people take videos of themselves doing this.”

Walking catfish eat native prey, although it isn’t known what impact they have on native fish. They are extremely sensitive to cold weather. “They just shut down even when we had a high of 70 degrees Fahrenheit,” Bressman says.

However, they have expanded into North Florida. Could they move farther?

“With their ability to move across land and access storm drains, they have the ability to spread quickly,” says Bressman. “Right now, colder temperatures are limiting their spread. But climate change could potentially make other southern states more amenable to walking catfish. Once they get in a system, they’re very difficult to control.”

Despite their invasiveness, Bressman appreciates the fish for what it is. Its ability to move around, on land, using a sense of smell and/or taste, is unrecorded in fish. And there’s much more to understand about this fish out of water.

“We still don’t know how this chemoreception works,” he says. “In many ways, this is just the tip of the walking catfish iceberg.”

Read the full journal article.